1. Technical Field
The solution according to the present disclosure relates to the electronic field. Particularly, such solution concerns switching direct current-direct current (dc-dc) converters.
2. Description of the Related Art
In electronics, a dc-dc converter is an electronic circuit adapted to convert a dc source from a voltage value to another one. An important application field of the dc-dc converters regards the electronic systems supplied through the power grid. Particularly, such electronic systems generally comprise a supply circuit capable of generating a dc voltage by rectifying the (alternating) voltage provided by the power grid; however, a generic electronic system is typically formed by a plurality of sub-circuits, each needing a different supply voltage value. The presence of one or more dc-dc converters allows to locally generate said different supply voltage values starting from the one generated by the supply circuit.
Among the various dc-dc converters presently available on the market, a well-know class thereof is represented by the so-called switching dc-dc converters. A switching dc-dc converter includes one or more switching elements (such as power MOS transistors) which are properly switched for generating a square wave starting from the supply voltage generated by the supply circuit.
Making reference to the switching dc-dc converters implemented according to the so-called half bridge configuration (briefly referred to as half-bridge converters), the switching elements include a high-side transistor and a low-side transistor connected in series between the supply circuit providing the supply voltage to be converted and a terminal providing a reference voltage, such as ground. By properly switching said two transistors it is possible to generate a square wave having a high value (assumed when the high-side transistor is activated) corresponding to the supply voltage and a low value (assumed when the low-side transistor is activated) corresponding to the ground.
Said square wave is provided to the primary winding of a transformer through a dc blocking capacitor; the secondary winding of said transformer feeds a rectifier circuit and a filter circuit for providing an output dc voltage. The value of the output dc voltage depends on the frequency and/or the duty cycle with which the switching elements are switched.
The switching frequency of the switching elements is significantly higher than that of the alternating voltage provided by the power grid. Therefore, the transformer included in a half-bridge converter can have a smaller size with respect to that of a transformer designed to be directly fed by the power grid. Moreover, a half-bridge converter is characterized by high efficiency, and low generation of heat.
However, while operating at higher frequencies allows a considerable reduction in the size of the passive components included in a half-bridge converter—such as the transformers and the filters—, a high switching frequency entails an increase of the so-called driving losses and of the so-called switching losses. While the driving losses are caused by the electrical power used for switching the switching elements, there are two different types of switching losses. A first type of switching loss is given by the passage of current through the switching elements occurring during the switching thereof; indeed, since the switching is not instantaneous, during a short period of time each switching element is crossed by a current when a voltage difference is developed across its terminal. The second type of switching loss is caused by the parasitic capacitance associated with each switching element, which discharges on the resistance of the switching element itself while the latter is activated. Both the driving and the switching losses are proportional to the frequency with which the switching elements are commuted. The switching losses become particularly noticeable in case it is necessary to provide electric charge for turning on a switching element implemented with a power MOS transistor that has a high drain to source voltage (“hard switching” condition).
To reduce the switching losses and allow high frequency operation, resonant conversion techniques have been widely developed. These techniques provides for processing electrical power in a sinusoidal manner, and controlling the switching elements in such a way to limit the occurrence of hard switching.
A half-bridge converter that exploits a resonant technique, referred to as resonant half-bridge converter, is provided with an input resonant network coupled with the primary winding of the transformer, in such a way that the network formed by the input resonant network and the primary winding of the transformer acts as a resonant tank.
Among the various known topologies of half-bridge converters exploiting a resonant technique, the so-called LLC topology is especially suited for those applications in which the value of the dc voltage to be converted is particularly high, i.e., in a condition favorable for the occurrence of high switching losses. The input resonant network of an LLC half-bridge converter is formed by a series LC circuit connected between the switching elements and an input of the primary winding of the transformer, and a shunt inductor connected across both the inputs of the primary winding; the series LC circuit may be implemented by connecting a series inductor to the dc blocking capacitor of the converter.
With an LLC half-bridge converter, it is possible to adjust the value of the output dc voltage over wide load and input dc voltage variations with a relatively small variation of the switching frequency. Moreover, the LLC topology allows to achieve a Zero Voltage Switching (ZVS) condition—wherein the power MOS transistors forming the switching elements switch at a nearly zero drain to source voltage—with ease. Particularly, by properly designing the input resonant network in such a way that the impedance's reactive component of the resonant tank is inductive for a sufficiently large switching frequency range, the current flowing into the resonant tank lags the voltage square wave generated by the switching elements.
In detail, during each falling edge of the square wave—and particularly at the time the low-side transistor is switched on—, said current is sunk by the resonant tank, while, during each rising edge of the square wave—and particularly at the time the high-side transistor is switched on—, said current is sourced by the resonant tank. As a consequence, every time the high-side transistor or the low-side transistor have to switch on, the drain to source parasitic capacitances associated therewith are already charged/discharge by the current output/sunk by the resonant tank, strongly favoring the occurrence of the ZVS condition.
It has to be appreciated that the resonant behavior can be also achieved without the presence of a dedicated input resonant network. Indeed, considering that the half-bridge converter already comprises a capacitor (i.e., the dc blocking capacitor), instead of providing dedicated series and shunt inductors, similar results can be achieved by substituting one or both of said inductors with corresponding parasitic inductances of a proper designed transformer (in this case, the transformer is referred to as “resonant transformer”).
The half-bridge converters are affected by a quite serious drawback occurring during the start up. Particularly, in the steady state, the voltage across the terminals of the capacitor included in the input resonant network comprises a dc component, corresponding to about half the supply voltage provided by the supply circuit, and an ac component that follows the course in time of the square wave. Since the capacitor blocks the dc component of such voltage, the voltage across the primary winding of the transformer exhibits the ac component only; as a consequence, in the steady state, the value of the magnetic flux linking the primary winding with the secondary winding of the transformer oscillates within a symmetrical range defined by such ac component only. On the contrary, at the start up of the converter, the capacitor is discharged; thus, when the high-side transistor switches on for the first time, the voltage seen by the primary winding performs a wide transition, from the ground to the high value of the square wave, i.e., the supply voltage generated by the supply circuit. As a consequence, the transformer works in the so-called “flux doubling” condition, with the magnetic flux that oscillates within a non-symmetrical range. This is a disadvantageous condition, which causes an asymmetry in the current sourced/sunk by the resonant tank. Particularly, since at the start-up the capacitor is completely discharged, when the high-side transistor is turned on for the first time, the current flowing into the resonant tank increases more rapidly than it decreases when the low-side transistor is turned on. As a consequence, when the high-side transistor is turned on at the subsequent cycle, the current is still positive, i.e., being still sunk by the resonant tank. In this condition, the high-side transistor is switched on in a hard switching condition, with the body diode of the high-side transistor that is crossed by current when the low-side transistor is turned on. As a consequence, the high-side and the low-side transistors are conductive at the same time, short-circuiting the terminal providing the supply voltage to be converted with the terminal providing the ground voltage. In this condition, a high peak of current is generated, which wastes a high amount of power. More importantly, in response to said high peak of current, the voltages across the terminals of the transistors may rapidly vary at a rate such to trigger the parasitic SCRs associated therewith.